The basic electrochemistry of the lead-acid battery was discovered over 140 years ago. It is relatively simple and well documented. The active materials involved in the battery chemistry are shown in Equation 1 below.Pb+PbO2+2H2SO4←→2PbSO4+2H2O  (Eq. 1)
Lead-acid batteries are widely used worldwide due to their low cost, robustness and acceptable energy density. However, they are heavy, and contain aqueous sulfuric acid which is corrosive. The lead-acid battery is inherently heavy because the anode and cathode contain high density lead and lead-based materials. The lead-based materials are generally formed into a “paste” which is held in a rigid “grid” structure for support. The paste is shaped into a plate supported by the grid substrate. The paste is subsequently treated and cured to form a high surface area “sponge” lead in the negative plate and lead dioxide in the positive plate. For example, U.S. Pat. No. 7,517,370 describes a method for making such a paste. As the lead-based grid structure adds considerable weight to the battery, various efforts to reduce grid/plate weight have been undertaken. Only about 50% of the active materials are actively involved in the electrochemical reactions, further compounding efforts to reduce overall battery weight.
U.S. Pat. No. 4,707,911 discloses a self-supporting porous electrode, without plates or grids to achieve a lighter battery. The electrodes are made by adding a leachable component to a molten lead mixture to form a solid in the required configuration, and removing the leachable component to create a porous electrode with a network of relatively uniformly distributed voids. The walls of the pores are then oxidized to form the electrochemically active material.
U.S. Pat. No. 4,713,306 describes a battery with at least a portion of the positive plate coated with electrically conductive doped tin oxide, where the positive plate can be in the form of organic or inorganic fibers in, for example, woven form. The tin oxide coating increases the conductivity. Such design enhances battery performance by allowing some conductivity to the non-conducting lead sulfate active material. U.S. Pat. No. 7,060,391 teaches the use of a reticulate carbon substrate having pores and a lead-tin containing alloy applied to such a substrate.
U.S. Pat. No. 5,156,935 describes the use of electrically conductive “wiskers” selected from carbon, graphite or potassium titanate mixed within the active materials of the negative and positive electrodes to provide lower weight, charge and volume efficiency, and improved resistance to sulfation.
U.S. Pat. No. 5,296,261 describes a method for producing a sponge metal made from nickel, copper or lead using a polymer sponge as a template which is impregnated with a solution of nitrite or sulfate of the metal. The object of the invention was to create a microporous metal structure for use as an electrode in portable alkaline storage cells. No reference is made to the electrochemical or corrosion protection of such an electrode in a highly oxidizing and corrosive environment, as is present in a lead-acid battery, nor is there any reference to the coating of such a metal substrate with any active materials.
U.S. Pat. No. 6,232,017 describes a lead-acid battery grid design that reduces the total weight of the battery by providing for an electrode grid comprising a reticulate made of an organic or inorganic compound (i.e. a sheet of glass fibers). Such a design allows for a battery weight reduction, especially for the negative electrode. A lead or lead alloy electricity collecting part covers part of the reticulate, where the grid can be covered with a thin film of a lead alloy on the reticulate surface.
U.S. Pat. No. 6,617,071 teaches a method whereby the battery plate is covered with a conductive polymer which is then coated with nanoscale particles of active material. Such design serves to reduce the weight of the battery and minimize positive grid corrosion. The design provides for the substantial utilization of the active material, as the nanoparticles are entrained and held within the conductive polymer skeleton and provide a large surface area for reaction. The conductive polymer has a rigid porous structure to promote the diffusion of acid and ionic species, but is still able to act as a “spring” during charging/discharging as the active materials change volume. However, the disclosed approach still requires a grid plate having at least one surface. Further, the conductive polymer skeleton limits the size of the active particles held therein to “nanoscale size”, thereby greatly limiting the mass of held particles, and thus the energy density of this design. The small size of the porous polymer structure limits the diffusion of the acid and ionic species, with the potential of the particles clogging the nano-sized pores during the discharge cycle, thereby limiting the rate of charge/discharge. Additionally, since the nanoparticles are added to the polymer after it is polymerized on the battery grid, the depth to which the nanoparticles can be entrained is greatly limited by the nano-sized pores within the polymer.
US Patent Publication No. 2009/0269658 A1 proposes a grid structure comprised of a low density material such as acrylonitrile butadiene styrene, which is then coated with a metal such as copper or nickel, followed by a coating of lead/lead alloy, followed by electrodepositing a layer of a conductive polymer such as polyanaline. However, such design still requires a grid/plate structure onto which the active and protective layers are coated. No reference is made to dealing with the reactive materials on the multi-coated substrate or the electrochemistry and reactions of the reactive materials.
To obtain reasonable energy storage capacity, porous positive and negative plates with high surface area are preferable. This can be achieved by applying a paste (ie. lead and lead oxide) over a lead-based grid. Such “pasted plates” are made porous by adding leachable materials to the active materials, which are subsequently removed. For example, U.S. Pat. No. 5,266,423 discloses the addition of magnesium, zinc or magnesium to the lead or lead-alloy, which is prepared by casing, then pulverized, with the additives finely dispersed in the lead or lead-alloy particles which were added to the paste. The additives are then leached by sulfuric acid to create a porous lead or lead-alloy matrix.
U.S. Pat. No. 5,047,300 describes the use of ultra-thin non-perforated electrode plates coated with an ultra-thin layer of active material with thin absorptive separator material layers. This arrangement provides for a large ratio of plate surface to active material, low internal resistance and heat dissipation during discharge. However, over time, such plates are subject to clogging of the pores by growth of non-conductive lead sulfate crystals. Such pore-clogging limits the rate of charge and discharge, and reduces cycle lifetime. These non-conductive crystals continue to grow and aggregate over repetitive discharge cycles, becoming increasingly difficult to convert back to lead-dioxide during the charge cycle. The sulfuric acid electrolyte becomes unable to penetrate the now-clogged pores and the electrical conductivity to the grid (collector) plate is lost, increasing the internal resistance and slowing the charge rate. This so-called “sulfation” process is a key limiting factor in battery charge/discharge capacity, ultimately causing the battery to fail. Various techniques have been developed to limit battery sulfation. For example, U.S. Pat. No. 7,592,094 B2 discloses a method for introducing mechanical excitation into the battery to minimize sulfation. That patent provides an extensive review of the prior art, which relate to methods for inducing electrical pulses to remove or reverse the sulfation process.
U.S. Pat. No. 6,979,513 describes a battery current collector having a carbon foam made from a carbonized wood substrate. The active materials are disposed on the carbon foam collector. No reference is made to protecting the surface of the carbon foam. Accordingly, such a design is prone to corrosion of the carbon material in the highly oxidative and acidic conditions at the positive electrode of a lead-acid battery, resulting in CO2 generation, and gradual loss of the carbon foam support structure during repetitive charge/discharge cycles. Additionally, this patent teaches the use of a paste or slurry of the active materials applied to the current collector, using a paste-containing transfer sheet, dipping or painting method, which approaches are limited to coating relatively large pores to a limited depth, where the foam collector acts essentially as a grid support for the active material paste or slurry.
US Patent Publication No. 2009/0269666 A1 discloses an electrochemical cell, including a lead-acid battery cell, where the carbon fiber structures are used to fabricate a grid/current collector to achieve weight reduction of the battery and to increase charging rate using a large surface area to active material. This patent teaches the use of a buffer layer between the graphite fibers and active materials where said buffer layer is comprised of a ternary carbide (ie. Ti3SiC2 or Ti2PbC), or noble metals such as lead, gold, silver, tantalum, platinum, palladium or rhodium. In one embodiment, thin current collecting plates are comprised of the carbide alloy and are reinforced with graphite fibers and active materials electroplated on each side. The approach still requires a current collecting plate and active materials which are electroplated only onto the surface of the grid. No reference is made to active materials deposited to or held within a 3-dimensional porous lattice structure.
Another salient issue of lead-acid battery chemistry is that the active materials at the positive and negative plates change dimensions in the x-y and z planes during the charge and discharge cycle. During the discharge cycle, the positive plate active material (lead-dioxide) converts to lead sulfate, where the lead sulfate structure expands by about 92%. Similarly, the negative plate active material (pure lead) also converts to lead sulfate on discharge, expanding by about 164%. During the charge cycle, the lead sulfate at both the positive and negative plate contracts back to lead-dioxide and pure lead, respectively. Such physical expansion-contraction of the plates create mechanical stresses within the battery, weakening the adhesion of the active material in the grid, and reducing the electrical continuity.
The lead-acid battery contains an aqueous solution of corrosive sulfuric acid of about 33% (v/v) at full charge. The sulfuric acid acts as an ionic conductive electrolyte and also takes part in the charge/discharge electrochemical reaction. Since the voltages required for complete battery re-charge are above that for the hydrolysis of water, oxygen is generated at the positive plate and hydrogen at the negative plate. The battery thus requires gas venting and servicing to replace lost water. This “open” gas system is generally referred to as the “flooded design”, which requires that the battery be maintained upright. The combination of the corrosive acid, and generation of oxygen and hydrogen during the charge cycle, represent a safety hazard. To resolve this issue, extensive efforts have been dedicated to fabricate so-called Valve-Regulated Lead-Acid Batteries (VRLA) containing a “fumed silica gel-acid mix” or “adsorptive glass fiber mat (AGM)” separators to entrain the corrosive sulfuric acid. Such batteries are sealed, and can be used in any orientation. U.S. Pat. No. 3,862,861 is the first one to disclose a method for fabricating such a sealed, maintenance-free lead-acid cell by utilizing an “oxygen-cycle”.
The separator is a key component in the VRLA battery, determining properties such as oxygen transport, electrolyte distribution and plate expansion. Extensive work has been done to optimize such separators. For example, U.S. Pat. No. 6,689,509 describes a multilayer separator comprised of a microporous polymer layer and at least one glass fibrous layer to provide for improved tensile strength and oxygen transfer. U.S. Pat. No. 5,128,218 describes a separator comprised of coarse and fine particles of hydrous silicon dioxide. Such a design creates a large specific surface area (20-400 m2/g), a porosity of 85-90%, with gas channels that permit efficient reaction for oxygen absorption.
A detailed discussed of VRLA-AGM separators is given in Chapter 7, pages 183-204, in an edited book by D. A. J. Rand, P. T. Moseley, J. Garche and C. D. Parker, titled “Valve-regulated Lead-Acid Batteries, 2004, Elsevier).
A key feature of sealed batteries is that they have a safety release pressure valve, and a mechanism for allowing oxygen and hydrogen to recombine. In the AGM design, such recombination occurs because the glass microfiber separator can adsorb large amounts of electrolyte and at the same time maintain some porosity, such that about 95% is electrolyte, with the balance void space available for rapid gas transport. Rapid oxygen transfer is not viable directly through the aqueous electrolyte due the slow diffusion rate of gas through fluid electrolyte. During charging, the electrogenerated oxygen can diffuse from the positive plate to the negative plate for oxidation with (unreacted) pure lead and for recombination with hydrogen (if excessively generated under conditions of overcharge). Use of an excess of active material at the negative plate will mitigate the generation of hydrogen. Also, use of purified active materials tends to suppress the oxygen cycle.
It is an object of the present invention to provide an improved lead-acid battery. The electrode according to the invention avoids the use of a grid or support plate and the active materials are substantially converted during each charge and discharge cycle. The invention reduces the overall battery weight, provides increased specific energy, minimizes sulfation, increases the rate of charge/discharge, decreases acid stratification and increases cycle lifetime as compared to typical prior art lead-acid batteries.
These and other objects of the invention will be appreciated by reference to the summary of the invention and to the detailed description of the preferred embodiment that follow, it being understood that not all objects are necessarily simultaneously attained by each aspect of the invention, and that not all objects are necessarily fulfilled by each claim of the application.